Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 283we cannot demonstrate the regional extent of the S 2fabric in this area (Fig. 8h).The third area is located in the northern Moldanubiandomain (Figs 2 & 6) and is characterized bythrusting of a NNW–SSE-elongated granulite bodyover the middle crust (Fig. 6a, c). The granulite showsan exceptionally well-preserved steep NNW–SSEtrendinggranulite-to-amphibolite facies S 2 foliation(Fig. 6, stereoplot A). D 3 shallow to moderately steep,south-dipping shear zones, which show top to theNNE shear sense, cut the S 2 fabric heterogeneously.The surrounding migmatites and migmatitic orthogneissesexhibit predominantly S 3 fabric, which dipsgently to moderately to the south (Fig. 6, stereoplotB). This foliation, formed essentially by migmatiticbanding, contains numerous close to isoclinal F 3 foldswith hinges trending NNE–SSW parallel to minerallineation. Within the adjacent middle crust, thedeformation is also polyphase, comprising an earlyfoliation that dips steeply to the SW, which was reworkedby a schistosity that dips gently in the samedirection (Fig. 6, stereoplot C).Structural development of the Lugian domainThe northern part of the Lugian domain is characterizedby a central granulite belt surrounded on thewestern and eastern sides by strongly migmatized orthogneisses(Figs 7c & 8b). The granulite and theadjacent orthogneisses reveal the NNE–SSW-trendingvertical S 2 foliation parallel to the trend of the orogeniclower crust (Fig. 7, stereoplot A). This S 2 foliationcontains rootless folds formed by layers of metabasitethat preserve evidence of an early S 1 foliation (Sˇtípska´et al., 2004).East of the granulite belt, the orthogneisses andmigmatite show reworking of the vertical S 2 fabric bymoderately west-dipping S 3 foliation (Fig. 8g). The dipof the S 3 fabric progressively decreases eastwards towardsthe underlying Ordovician lower crustal leptyno-amphiboliteunit (Fig. 7, stereoplot B); agranodiorite sill emplaced syntectonic with the D 3deformation marks the boundary between this unitand the Lugian orthogneiss, dating the D 3 structures atc. 340 Ma (Parry et al., 1997; Sˇtípska´ et al., 2001).Importantly, the structure of the leptyno-amphiboliteunit is discordant with respect to D 3 fabrics of thewestern Lugian orthogneisses; the leptyno-amphiboliteunit is inferred to be of Early Ordovician age, based onU–Pb zircon dates that yield an age of c. 510 Ma(Fig. 7, cross-section; Sˇtı´ pska´ et al., 2001; Lexa et al.,2005).To the NW from the granulite belt, the S 2 fabric isaffected by sub-horizontal, variably dipping amphibolitefacies S 3 fabric (Fig. 8a,b). This S 3 fabric hasintensely reworked the steep S 2 foliation of the adjacentorogenic middle crust to the point that it progressivelypasses into a several hundred metre widenormal-sense shear zone dipping gently to the NE(Fig. 8c).P–T–t history of the Moldanubian–Lugian rootThe relationship between the prograde andretrograde P–T paths and orogenic fabrics is a keypiece of information necessary to understand thethermo-mechanical processes that operated withinthe orogenic root. Microstructural, petrological andthermodynamic modelling studies during the lastfive years have revealed a systematic pattern of P–Tevolution related to the early steep fabrics in boththe orogenic lower and middle crust. In addition,combination of the P–T evolution with Sm–Nd and40 Ar ⁄ 39 Ar cooling ages reveals important differencesin the thermo-chronological evolution of theorogen.Microstructural and petrographic characterization of S 2 andS 3 fabricsIn the orogenic lower crust, the steep fabric is characterizedby compositional banding formed by maficand felsic granulite (Sˇtı´ pska´ et al., 2004). More commonlyS 2 is defined by a mineral fabric, marked byoriented kyanite, biotite and recrystallized ribbons ofquartz and feldspar in the case of the felsic granulite(Tajcˇmanova´ et al., 2006), and by the elongated shapeof garnet and aligned omphacite grains in the case ofthe mafic granulite (Sˇtı´ pska´ et al., 2004). In the orogeniclower crust composed of migmatitic orthogneisses,the S 2 fabric is defined by alternation of infinitemonomineralic recrystallized bands of quartz, plagioclaseand K-feldspar, in an assemblage with kyanite,biotite and garnet that results from a high-temperatureFig. 6. (a) Structural map of an area showing thrusting of the orogenic lower crust over the orogenic middle crustal in the NE part ofthe Moldanubian domain, with regional S 2 and S 3 fabrics as shown in Fig. 5 (after Tajčmanova´ et al., 2006). See Fig. 2 for regionallocation. The map shows a large body of orogenic lower crustal granulites that preserve relicts of the S 2 fabrics. The structuraltrends (thin layers – S 3 fabrics, thick layers – S 2 fabrics) indicate extrapolations of major orientations of structural fabrics in the field.The density of trend-lines indicates the homogeneity of fabric elements in the field. (b) Stereograms: A – stereogram shows poles to S 2fabrics in the orogenic lower crustal granulites; B – stereogram of poles to S 3 fabrics from the orogenic middle crust. Squares instereogram show integrated directions of L 3 lineations from the area. C – Stereogram of poles to S 3 fabrics from the orogeniclower crustal granulites and associated migmatites. Equal area projection, lower hemisphere, contoured at multiples of uniformdistribution. (c) Simplified cross-section shows well-preserved S 2 fabrics in competent granulites surrounded by cordierite migmatitescompletely reworked by S 3 fabric. The migmatites and granulites of the orogenic lower crust are thrust over orogenic middle crustto the east. The numbers on the cross-section and map show locations of samples used for P–T calculations in Fig. 9.Ó 2007 Blackwell Publishing Ltd143